1. Field of the Invention
The present invention concerns hydraulic steering of vehicles making use of hydraulic modules generating a rotary movement of variable amplitude derived from a constant hydraulic output.
The invention falls into the domain of self-propelled vehicles featuring at least two steerable wheels. More specifically, it concerns the field of self-propelled machines with two steerable wheels, allowing, for example, turning radii according to Jeantaud's diagram. It is especially applicable to: the equipment of grape harvesters; and vehicles or self-propelled machines with at least two steerable wheels, for which the users would like to have available machines capable of achieving the smallest turning radius possible.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Self-propelled machines with at least two steerable wheels are equipped with steering mechanisms which make it possible to steer them while they are in motion. These steering mechanisms act directly on each steerable wheel to obtain the turning radius of the machine.
In order to achieve this turning radius in an efficient manner and to avoid the phenomenon of “wheel slipping” (the wheels sliding instead of rolling, thereby degrading the soil as well as the tires), it is necessary, during the turn, that the rotating axes of each of the wheels of the machine, regardless of whether they are steerable or not, be concurrent in a single point which is the instantaneous center of rotation (ICR) of the machine in projection to the ground: this is the application of the principle in the Jeantaud diagram.
Various methods exist to obtain this diagram (mechanical, electric, hydraulic, even combined steering mechanisms), the most common one actuating steering tie-rods and jacks which make it possible, according to a geometry adapted and known by the experts, to approximate this diagram.
The major setbacks of this steering system are: the space requirement; a wide turning radius of the vehicle, because the maximum turning angle is limited b the technology of this tie-rod and jack system; and a very significant loss of torque at the steering lock due to the kinematics of this type of steering.
For a self-propelled machine of the type used, for example, in agriculture and in particular for a straddling vehicle (grape harvester, trimming machine, etc.), it is very important to increase the maneuverability of the machine, especially on short headlands, and to limit the number of maneuvers in steering operations. That is why one seeks to limit the turning radius of the machine to the smallest value possible.
But obtaining this small turning radius is not possible with the mechanical technologies existing today while respecting the Jeantaud diagram, from the straight line to the maximum turn, especially with machines equipped with connecting rod and cylinder steering systems or even with conventional rack cylinders.
Furthermore, in the case of a straddling machine, the travel lane of the vehicle is related to the distance between two plant rows, which varies depending on the region and the type of plants cultivated. If one plans to build a variable track vehicle, it is difficult to be satisfied with a mechanical steering system based on connecting rod assemblies and cylinders.
The builders of such vehicles are therefore intent on:                doing away with the mechanical linkages of the steering wheels;        being independent of the variability of the vehicle's driving lane;        being mindful of the Jeantaud diagram at any point during the turn; and        having at their disposal a system which allows them to reduce the turning radius to its possible minimum when this is necessary.        
Various solutions for steering systems exist today:                Steering system by mechanical rodding and cylinders: the oldest and today most conventionally used system. Generally the left and right rodding assemblies of such a system are mechanically linked.        Electric steering system: such as described for instance in document EP-0.300.185, applied to a wheel-chair for handicapped persons, where the two front wheels are coupled, each to a directional motor that is manually activated over a mechanical/electrical transmitter/calculator of the directional angle of the wheels.        Hydraulic steering system: by way of, for example, a hydraulic cylinder system with a straight rack, as described in documents FR1.201.675 or FR-2.694.535 describing a hydraulic transmission by hydraulic cylinder with a piston rod positioned at each of the steering wheels;        Mechanical steering system with a dual rotary steering cam as described in the US-2007/01 44796 document.        
Among the systems cited above, the electrical steering, system as described for the wheel-chair mentioned in the EP-0.300.185 document, presents several advantages to meet the requirements at hand, but when one speaks of a vehicle weighing several hundreds of kilograms or even several tons, the electrical systems to deploy (steering motors, electrical drive motors for the wheels, etc.), are certainly feasible with present technologies, but those are complex and expensive solutions, because the currents needed require the set-up of power electronics that are complex and still somewhat unreliable. This is under consideration for all-electric vehicles with only limited range.
As to mechanical steering systems, rodding systems are excluded with respect to the objectives aimed for by the invention and the dual rotary cam systems may well provide solutions for light vehicles such as riding mowers, but they are hardly practical due their complexity on a straddling vehicle that is also capable of adapting to a variety of terrains.
Also, on a straddling, vehicle where traction on the wheels is achieved by hydraulic motors, it would be desirable to make use of the same energy to steer those same wheels.
The hydraulic steering systems that is proposed in document FR2.694.535 is nevertheless not satisfactory because it completes the hydraulic solution with a mechanical rodding assembly at each wheel and does not allow, taking into account the space requirements of the cylinders and said mechanical rodding, to easily achieve turning angles above 90 degrees.
Documents WO-01/25071 and U.S. Pat. No. 2,757,014 describe hydraulic steering systems of prior art.
In the other domains, for the most part already mentioned such as light vehicles (wheel-chairs, riding mowers, etc.) the latter make reference rather to electric or mechanical steering systems for farm or utility, or even military, vehicles.
Apart from the application in the area of light vehicles or that of articulated vehicles with more than two steering vehicles (forest vehicles for example), there is no known hydraulic steering system capable of satisfying the demands for enabling turns with a very small value of turning radius. The aim of the invention is indeed to remedy the drawbacks and insufficiencies of steering systems at the state of the art.